This invention relates to an ion exchange material for the removal of radioisotope cations from an aqueous environment, which comprises a modified clinoptilolite, and to a method for the removal of radioisotope cations from an aqueous environment using the modified clinoptilolite.
The successful treatment of low level radioactive effluent represents a major challenge to the nuclear industry at the present time. Such effluent can take the form of low level radioactive waste waters created by nuclear reactors, which contain the radioisotopes of strontium, .sup.90 Sr, and caesium, .sup.137 Cs, as their cations. The effluent alternatively could take the form of low and middle level radioactive waste in solid form which has to be encased or entombed in such a way as to ensure that leaking of radioisotopes by water (e.g. .sup.90 Sr or .sup.137 Cs in their cation forms) does not occur or is retarded. There exists therefore a need to identify materials that can selectively remove the radioactive cations from the aqueous environments so that these radioactive waste materials can be made safer for disposal in the environment.
Previous studies have identified that the natural zeolite, clinoptilolite, (L. L. Ames Jnr, Amer. Mineral 1960, volume 45, pages 689-700) from the Hector deposit in the USA could be used to remove .sup.137 Cs.sup.+ and .sup.90 Sr.sup.2+ from the radioactive waste waters of nuclear reactors. More recently Mercer and Ames (in "Natural zeolites, occurence, properties and use", editors: L. B. Sand and F. A. Mumpton, Oxford, Pergamon, 1978pages 451-460) have disclosed that natural zeolites have been used in the treatment of high-level radioactive wastes to remove .sup.137 Cs.sup.+ ; decontamination of low and medium level wastes; and the fixation of fission products with zeolites prior to long term storage.
In general radioactive effluent treatment problems vary with reactor type and location, but all reactors face the problem of containment of fission products. A particular problem for the United Kingdom is the removal of fission products released into storage ponds during the handling and storage of spent fuel elements from nuclear power reactors. The ponds contain low levels of .sup.137 Cs.sup.+ and .sup.90 Sr.sup.2+, present at less than milligram/l concentrations, as a result of corrosion of the fuel elements. Clinoptilolite as the natural mineral, particularly that from Mud Hill deposit in California, USA, has been found to be particularly successful in removing these cations from the water (See A Dyer "Uses of Natural Zeolites", Chemistry and Industry, Apr. 2, 1984, pages 241-245, for a general discussion of this application).
Although natural mineral deposits for clinoptilolite have been found to be useful in this application the zeolite is not widely utilised due to the variability of composition that is apparent. However, a few natural clinoptilolites of uniform purity and composition have been located (e.g. some deposits from Nevada, USA) and these are utilised commercially.
It has also been known to use treated clinoptilolites for the removal of caesium and strontium cations from waste solutions. In an article entitled "Selective Sorption of Caesium-137 on a Clinoptilolite Modified by
Hexacyanoferrates (II), (G Goshev, K Daiev and A Aleksiev, Yad. Energ. 1987, volume 24, pages 74-81) there is disclosed the use of clinoptilolite modified with hexocyanoferrate for the removal of .sup.137 Cs form radioactive wastes. Further, in an article on Processing of Radioactive Waste Solutions with Zeolites, (H Mimura and T Kanno, Nippon Genshiryoku Gakkaishi, 1977, volume 19, No 3, pages 170-176) there is disclosed the use of a calcined natural zeolite such as clinoptilolite for the processing of radioactive waste solutions containing caesium and strontium.
Zeolites used in this application take advantage of their known resistance to radiation, in contrast to ion exchange resins which tend to lose capacity and selectivity when exposed to radiation. Zeolites also are compatible with the long term storage and disposal techniques used in the nuclear industry, since spent ion-exchange materials are usually encapsulated in cement or glass prior to long term storage or dumping of these low level wastes at sea. In addition zeolites are useful as barrier materials in the storage and containment of high level wastes when buried in trenches or underground caverns. When utilised as a barrier material the zeolite acts as an absorbent for radioisotopes that leak from the high level waste due to failure of the encapsulation media.
While natural zeolites can be used in these applications there is a continual need to identify improved materials that can be more effective in the treatment of radioactive effluent.